The Mechanism Of Metal Ions Affecting The Oxidation And Photolysis Of Antibiotics Under Combined Pollution

Antibiotics,a kind of widely used effective antimicrobial drugs,were highly detected in the water environment.Due to their "pseudo-persistence" and easy induction of resistance genes,the potential environmental risks of antibiotics have got a great attention.However,there is not just the single antibiotic pollution in the natural environment.With the addition of some metal elements in animal feeds,the fertilizer use of livestock and poultry manure,and the sewage irrigation,the combined pollution of metal-antibiotic has become an important pollution characteristic of antibiotic pollution the water environment.The biotransformation contributes little to the transformation of antibiotics because of the inhibition,and oxidation and photolysis of antibiotics become the important ways of natural reduction in water environment.Antibiotics contain several ionizable functional groups,theoretically providing lone pair electrons complexation with metal ions,which influence both of environmental behavior and ecological effect under single pollution condition.However,most of the existing research of didn't consider the interaction of the two pollutants.There was little study on the mechanism of metal ions affecting on the transformation of antibiotics.Therefore,The study of the interaction mechanism between metal ions and antibiotics under combined pollution,as well as the mechanism of metal ions affecting on the environmental behaviors of oxidation and photolysis of antibiotics,helping us to grasp their migration and transformation,to accurately evaluate the environmental risks in the water environment,and to provide theoretical basis for formulating more reasonable pollution control measures.In this study,two kind of antibiotics with different structure,characteristics and high detection rate(tetracycline(TC)and sulfamethazine(SMT)),and four metal ions(Fe(?),Mn(?),As(?)and Cr(?))were set as the main study object.The theoretical calculation of complexation model and batch experiments were proceeded to study the complexation behavior of different antibiotics and metal ions,to investigate the oxidation and photolysis behavior of antibiotics induced by different metal ions and to illuminate mechanism of metal ions affecting on the oxidation and photolysis of antibiotics.The main conclusions were as follows(1)The distribution of metal cations(Fe(?)and Mn(?))and antibiotic(TC and SMT)species,as well as the complexation species and mechanism between them were analyzed by the theoretical calculation of surface complexation model.For TC,the main species were H2TC and H3TC+ under acidic conditions.With the increasing of pH,the proportion of deprotonated species increased,producing HTC-and TC2-.For SMT,the main species were SMT and SMT+under acidic conditions,as the pH increasing,the deprotonation of SMT occurred,producing SMT".The complexation ability of metal cations Fe(?)and Mn(?)with TC was far greater than others(Cd(?),Zn(?)and Pb(?)),which are mainly existed as Fe(H2TC)23+ or FeHTC2+and MnTC or MnHTC+,respectively.However,under the same reaction conditions,SMT complexed weakly with metal cations,which could hardly affect the species distribution for each other.Thus,the structure characteristics of functional groups and dissociation constants of different antibiotics,as well as the complexation constants and equilibrium reactions with different metal ions determined their species and abilities of complexation.(2)The kinetics,influencing factors and mechanism of TC and SMT photolysis under simulated light irradiation were studied.The protonation state and absorption spectrum at different pHs affected the photolysis of TC and SMT.With the increasing of HTC-and TC2-species under higher pH conditions,the photolysis rate of TC increased.The photolysis rates of ionic species(SMT+ and SMT-)were greater than SMT species.The influencing factors in the natural water environment had different effects on the photolysis of TC and SMT.Fe(?)could promote the photolysis of TC,but hardly affecting photolysis the of SMT,which is related to the complexation ability.The presence of oxygen molecules promoted the photolysis of TC but inhibited that of SMT,indicating that the self-sensitization oxidation rate of SMT was lower than the direct photolysis.And the production of the singlet oxygen(1O2)was related to the self-sensitization oxidation of TC and SMT,promoting the photolysis.Based on the calculation of reactive sites and the detection of products,the photolysis of TC mainly included hydrolysis and hydroxylation.And the photolysis of SMT included hydroxylation,SO2 extrusion and S-N bond cleavage.(3)The photolysis mechanism of SMT/TC with low molecular weight organic acids/As(?)induced by Fe(?)was investigated.It was found that the complexation between Fe(?)and three organic acids(citric acid(CA),tartaric acid(TA)and malic acid(CA))could promote the photolysis of SMT.An interaction among ROS(reactive oxygen species),Fe(?)-carboxylate complexes and SMT was proposed.The presence of SMT did not affect the complexation species,which was beneficial to induce efficient photolysis of SMT.Enough hydroxyl radicals(·OH)were produced and 1O2 was originated from the self-sensitization oxidation of SMT.All functional groups underwent synchronously changes during photolysis,but the degradation rates of functional groups were different.The transformation pathway of SMT mainly included hydroxylation,SO2 extrusion,Smiles-type rearrangement and S-N bond cleavage.Therefore,the presence of Fe(?)and organic acid complexes accelerated the photoconversion process of SMT in the environment,breaking the limitation of the weak interaction between Fe(?)and SMT.However,As(?)inhibited the photolysis of TC induced by Fe(?),and the interaction mechanism between Fe(?),As(?),TC and ROS under light irradiation was proposed.The concentration of Fe(?),As(?)and TC decreased,and the species of[Fe(H2AsO3)]2+,[Fe(H2AsO3)2]+and FeHTC2+gradually decreased due to photolysis and oxidation,while the contents of Fe(II)and As(V)gradually increased.The contributions of Fe(?),·OH and superoxide anion(O2·-)to photolysis of TC were 55.6%,20.5%and 6.8%at pH 3.0.The contributions were 68.3%,4.7%and 2.2%at pH 6.5,respectively.The intensity of free radical was weak under the neutral condition.During the reaction,TC competed with As(?)for photons,ROS and Fe(?).The transformation pathways of TC mainly included hydrolysis,hydroxylation,demethylation and deamination(only at pH 6.5)(4)The mechanism of heavy metal ions of Mn(?)and Cr(?)affecting on the oxidation and photolysis of TC was studied.It was found that the oxidation of TC could be induced by Mn(?)and Cr(?)without light irradiation.However,the effects of those two metals on TC photolysis under simulated light irradiation were different.Mn(?)could accelerate the oxidation of TC and inhibit the photolysis of TC.Firstly,Mn(?)complex with TC,forming TC-Mn(?).The complex was oxidized by the dissolved oxygen to generate Mn(?).Due to the existence of electrical pair between TC-Mn(?)and TC-Mn(?),Mn(?)oxidized TC to form the oxidative degradation product,while Mn(?)was reduced to Mn(?).Mn(?)acted as a catalytic role in promoting degradation of TC.For the photolysis of TC with Mn(?),Mn(?)had a quenching effect on TC static fluorescence spectrum,and excited states of TC could compete with energy dissipative reaction.Based on the oxidation degradation,complexation and electron transfer process,the energy of excited TC*transferred and the bond breaking reaction was blocked.Subsequently,the photolysis reaction was inhibited.During the reaction,with the significantly changing of all functional groups,the band strength decreased,and the degradation rate of functional groups was different.The transformation pathways of TC mainly included hydrolysis,hydroxylation,deamination,dehydration,decarbonylation,demethylation,decarboxylation and ring-opening reaction.The anion Cr(?)could promote the oxidation and photolysis of TC.The electron-transfer interaction among 1O2 from self-sensitization,Cr(?)/Cr(?)and TC was proposed.When Cr(?)oxidized TC to form Cr(?)and TC radical,Cr(?)complexed with TC to accelerate the oxidation.In addition,the Cr(?)-induced oxidation of TC still occurred under simulated irradiation.With the increase of Cr(?)concentration and pH value,the intensity of 1O2 increased.The photolysis of TC was associated with self-sensitized oxidation processes induced by 1O2,In the presence of Cr(?),the formation of products and self-sensitization of TC was affected.Under simulated UV irradiation,the contribution of Cr(?)was 30.3%,1O2 21.5%and O2·-11.7%.Under simulated sunlight irradiation,the contribution of Cr(?)was 34.0%,1O2 9.7%and O2·-9.6%The transformation pathway of TC mainly included five reactions:hydrolysis,hydroxylation,deamination,dehydration and demethylation only for photolysisIn summary,the presence of widespread metal ions in water environment significantly influence the oxidation and photolysis behavior of antibiotics,which is a simultaneous complex process involving in complexation,redox cycle for metal ions,antibiotic conversion decomposition(oxidation and photolysis),and multipath electron transfer The results of this research can provide theoretical supports for the accurate assessment of the migration and transformation behaviors,environmental fates and ecological risks of metal ions and antibiotics under the combined pollution.